Julie Diani

Directional model for isotropic and anisotropic hyperelastic rubber-like materials

A material direction-dependent constitutive model has been formulated for large deformations for isotropic and anisotropic rubber-like materials. Although such materials are usually isotropic, anisotropic behavior has been observed in calendered plates of filled rubbers. Strain energy density function characterizing rubber-like materials is usually dependent on principal stretch ratios and thus is unable to account for anisotropy, whereas the proposed strain energy density depends on material directions and accounts for anisotropy. The material directions have simply been chosen using regular solid geometry. The strain energy density is given as the sum, over all material directions, of elementary directional strain energy densities. Then the elementary strain energy form is phenomenologically determined to account for the state of strain dependence of the material response. The model response is compared to uniaxial tension experimental data for anisotropic hyperelastic rubber-like materials and to uniaxial and biaxial tension for isotropic rubber-like materials.

Molecular dynamics simulations of the shape-memory behaviour of polyisoprene

Full-atomistic molecular dynamics simulations are used to study the shape-memory behaviour of a representative amorphous polymer. A virtual polyisoprene was constructed and subjected to uniaxial stretch and hydrostatic compression thermomechanical cycles. Uniaxial stretch loading results demonstrate that a temporary shape can be stored at low temperature and that the original shape can be recovered at high temperature due to the entropy effect involved in a change of shape at high temperature. The material volume change during pure hydrostatic loading resulted in a change of internal energy. The volume change could not be stored at low temperature without applying hydrostatic stresses. These results shed light on the fundamental mechanisms driving shape memory and recovery in amorphous polymers.

CHARACTERIZATION OF THE MULLINS EFFECT OF CARBON-BLACK FILLED RUBBERS

Several carbon-black filled styrene-butadiene rubbers showed different sensibilities to the Mullins softening when submitted to cyclic uniaxial tension. In order to quantify this softening, a damage parameter was introduced. It is defined by using a classic damage approach and can be estimated by using either the strain amplification factor method or the tangent modulus at zero stress. The proposed parameter is used to study the effects of crosslink density and filler amount on the Mullins softening. The latter is shown to remain unaffected by a change of crosslink density and to increase with an increase of filler amount. The damage parameter exhibits mere linear dependences on the maximum Hencky strain applied and on the filler volume fraction. A simple linear expression is given finally to predict the Mullins softening of filled rubbers. The parameter also provides an objective analysis for the Mullins softening that supports comments on a better understanding of this effect.

Experimental and modelling studies of the shape memory properties of amorphous polymer network composites

Shape memory polymer composites (SMPCs) have become an important way to leverage improvements in the development of applications featuring shape memory polymers (SMPs). In this study, an amorphous SMP matrix has been filled with different types of reinforcements. An experimental set of results is presented and then compared to three-dimensional (3D) finite-element simulations. Thermomechanical shape memory cycles were performed in uniaxial tension. The fillers effect was studied in stress-free and constrained-strain recoveries. Experimental observations indicate complete shape recovery and put in evidence the increased sensitivity of constrained length stress recoveries to the heating ramp on the tested composites. The simulations reproduced a simplified periodic reinforced composite and used a model for the matrix material that has been previously tested on regular SMPs. The latter combines viscoelasticity at finite strain and time-temperature superposition. The simulations easily allow representation of the recovery properties of a reinforced SMP.

Study on the temperature dependence of the bulk modulus of polyisoprene by molecular dynamics simulations

The temperature dependence of the bulk modulus of polyisoprene has been studied using molecular dynamics simulations. Virtual polyisoprenes have been submitted to volume contractions above and below the glass transition. Bulk modulus has been observed to be linearly dependent on temperature both above and below the glass transition respectively, and it dropped by a factor of about 2 while temperature was raised above the glass transition. By monitoring the energy changes during volume contractions, it was observed that the bulk modulus arises mainly from the Van der Waals interactions. Nevertheless, the entropy contribution to the bulk modulus becomes significant above the glass transition. At a first order, the entropy part of the bulk modulus can be considered as independent of the temperature.

Experimental study and numerical simulation of the vertical bounce of a polymer ball over a wide temperature range

The dependence to temperature of the rebound of a solid polymer ball on a rigid slab is investigated. An acrylate polymer ball is brought to a wide range of temperatures, covering its glass to rubbery transition, and let fall on a granite slab while the coefficient of restitution, duration of contact, and force history are measured experimentally. The ball fabrication is controlled in the lab, allowing the mechanical characterization of the material by classic dynamic mechanical analysis. Finite element simulations of the rebound at various temperatures are run, considering the material as viscoelastic and as satisfying a WLF equation for its time–temperature superposition property. A comparison between the experiments and the simulations shows the strong link between viscoelasticity and time–temperature superposition properties of the material and the bounce characteristics of the ball.

Effect of the Mullins softening on mode I fracture of carbon-black filled rubbers

The effect of the Mullins softening on mode I fracture of carbon-black filled rubbers was investigated experimentally. Large specimen of NR and SBR filled with the same amount and nature of carbon-black were submitted to uniaxial tension. Then, single edge notch tension samples were cut along various directions with respect to the direction of preconditioning, and submitted to tension until break. The fracture energy was estimated and compared according to the intensity of Mullins softening already undergone in the direction of crack opening and according to the softening undergone in other directions. The NR shows significantly improved resistance to crack propagation compared to the SBR due to its crystallization ability. For both materials, it was observed that a moderate prestrain has a positive impact increasing the material fracture toughness and that material softening and anisotropy induced by Mullins effect does not show on resistance to mode I crack propagation.

Relationship between microstructure and elastic properties of semi-crystalline polymers

Actually semi-crystalline materials are widely used as structural materials. During the part forming, the stretching or the shearing of the polymer melt under strong cooling conditions lead to generate specific crystalline morphologies such as deformed spherulites, shish– kebab or more complex crystalline macrostructure like in polypropylene for example [1]. Moreover crystallinity variations along the part and in the part depth can be observed. The crystalline orientation is responsible for possible anisotropic behavior while variations of the amount of crystallinity induce strong variations of the mechanical properties. There is an industrial need of developing behavior laws for the prediction of these mechanical properties. Actually, simulation based on molecular models allows the prediction of the final molecular orientation [2, 3, 4]. It stills a huge gap between predicted crystalline morphology of the polymer and the prediction of their mechanical properties. Even thought semicrystalline polymer are closer to composite even nano-compsite material few works are done to predict their properties as it the case of composite or filled polymer. Our work deals with te elastic properties. In his works of Halpin and Kardos [5] proposed determine the elastic moduli of semi-crystalline polymers. The lamellae are supposed to be fibers. An adjustable parameter in this model was linked to crystallite shape ratio. However, this model is well adapted for low volume fraction. This is not the case of semi-crystalline materials, for which the crystallinity can often reach 60 to 70%.